This method is used to resolve classnames throughout CakePHP. It resolves
the short form names CakePHP uses and returns the fully resolved classname:

// Resolve a short classname with the namespace + suffix.App::classname('Auth','Controller/Component','Component');// Returns Cake\Controller\Component\AuthComponent// Resolve a plugin name.App::classname('DebugKit.Toolbar','Controller/Component','Component');// Returns DebugKit\Controller\Component\ToolbarComponent// Names with \ in them will be returned unaltered.App::classname('App\Cache\ComboCache');// Returns App\Cache\ComboCache

When resolving classes, the App namespace will be tried, and if the
class does not exist the Cake namespace will be attempted. If both
classnames do not exist, false will be returned.

Ideally vendor files should be autoloaded with Composer, if you have vendor
files that cannot be autoloaded or installed with Composer you will need to use
require to load them.

If you cannot install a library with Composer, it is best to install each library in
a directory following Composer’s convention of vendor/$author/$package.
If you had a library called AcmeLib, you could install it into
vendor/Acme/AcmeLib. Assuming it did not use PSR-0 compatible classnames
you could autoload the classes within it using classmap in your
application’s composer.json: